A mysterious disease that the Center for Disease Control has only monitored for a few years set a new record for cases in the United States this year, according to the Associated Press. Acute flaccid myelitis, which causes paralysis in victims and only flares up every other year, has affected 158 people in the U.S. in 2018.

By the end of November, AFM had affected 116 people in the U.S. across more than 30 states. The polio-like illness weakens muscles by attacking the nervous system, resulting in temporary or even lasting paralysis. More than 90 percent of those affected are children, per the CDC.

It typically starts with respiratory problems and a fever before unleashing its worst symptoms around a week later.

Even though the symptoms are similar, the CDC has ruled out any links to polio. Even more mysteriously, the number of AFM cases in the U.S. surges every other year. Since the first outbreak in 2014, large outbreaks have occurred in 2016 and 2018, but not the odd-numbered years in between.

The same trend has not applied to the other countries where AFM has been reported.

One silver lining, according to the CDC, is that AFM has not killed anyone yet in 2018. One death in 2017 was attributed to AFM.